1. Field of the Invention
The present invention relates to broadband antenna apparatus for communication systems that need small UWB (ultra wide band) antenna apparatus. The communication systems may be broadband PAN (personal area network) systems using the UWB technology.
2. Description of Related Art
The implementation of a broadband PAN using the UWB technology needs a UWB antenna, which may be a disk monopole antenna.
A very general monopole antenna includes a flat conductor as a ground and a linear conductor as a radiating element. The size of the ground is roughly equal to the working wavelength. The size of the radiating element is about ¼ of the wavelength. The radiating element is set over the ground perpendicularly to it. An arbitrarily gap is formed between the ground and the radiating element, and electricity is supplied in the gap. This monopole antenna can operate in a frequency band lower than 20% of the central frequency. Accordingly, this antenna is unsuitable as it is for a UWB.
It is therefore proposed that the radiating conductor of a monopole antenna be a disk, which has very wide band characteristics. FIGS. 10A and 10B show a disk monopole antenna, which includes a radiating element in the form of a disk.
FIGS. 10A and 10B are a side view and a top plan respectively of a disk monopole antenna. This monopole antenna includes a conducting ground plate 100 and a radiating conductor 200 in the form of a disk. The radiating conductor 200 is set over the ground plate 100 substantially at right angles to it with a gap d between the plate 100 and the conductor 200. As shown in FIG. 10A, the disk monopole antenna has a ground feeding point 100f and a signal feeding point 200f. 
The lowest frequency of the frequency band in which the monopole antenna shown in FIGS. 10A and 10B can operate is the frequency equivalent to a wavelength that is about four times the diameter of the antenna. The highest frequency of this band is several times as high as the lowest frequency. FIG. 12 shows the VSWR (voltage standing wave ratio) characteristic of the monopole antenna shown in FIGS. 10A and 10B, with the radiating conductor 200 having a diameter h of 23.5 mm.
As shown in FIG. 12, the VSWR characteristic is stable over a wide band from about 3 GHz to 8 or more GHz. FIG. 12 confirms that the disk monopole antenna can be used in the wide band. The radiation directivity of the disk monopole antenna shown in FIGS. 10A and 10B is horizontally in-plane non-directional like ordinary monopole antennas.
FIGS. 11A and 11B are side views on the x-z and y-z planes respectively of a bent disk monopole antenna, and FIG. 11C is a top plan of this antenna, which is a modification lowered in height of the disk monopole antenna shown in FIGS. 10A and 10B.
The bent disk monopole antenna shown in FIGS. 11A–11C includes a conducting ground plate 100 and a radiating conductor 200 in the form of a disk. The radiating conductor 200 is set over the ground plate 100 substantially at right angles to it with a gap d between the plate 100 and the conductor 200. The upper half of the radiating conductor 200 is bent so that the height of this conductor is one half of that of the conductor 200 shown in FIGS. 10A and 10B. As shown in FIGS. 11A and 11B, the bent disk monopole antenna has a ground feeding point 100f and a signal feeding point 200f. 
As shown in FIG. 13, the VSWR characteristic of the bent disk monopole antenna shown in FIGS. 11A–11C is such that the lower limit of the frequency band in which the VSWR is 2 or lower is a little higher, but this band is still wider than the frequency band for ordinary monopole antennas. Accordingly, this antenna can be used as a low broadband antenna.
The disk monopole antenna and the bent disk monopole antenna are broadband antenna apparatus that may be used for the broadband PAN system employing the UWB technology. These antennas may still be too large in size to be mounted in or on equipment.
For this reason, it is desired to provide smaller broadband antenna apparatus that can operate in a frequency band not narrower than those for the conventional disk monopole antenna and the conventional bent disk monopole antenna.